A factor having major impact upon the propulsion efficiency of a vessel is the configuration of its hull. Many vessels are provided with a bow bulb in order to reduce hull resistance. The bulb is generally submerged when the vessel is fully loaded, but in some designs, especially in tankers, the bulb is designed so its upper periphery projects above the water-line, when the ship is in ballasted condition. There is a marked difference between the draft in ballast condition and in fully loaded condition, so even in a tanker the bulb will be well below the water-line, when the ship is loaded.
Many ships for general purposes do not encounter the same difference in draft, and inventor has in his U.S. Pat. No. 4,550,673 proposed a bulb, which, when the ship is in fully loaded condition and stationary, with its upper periphery, projects above the water-line. The idea is, that the oncoming water shall be lifted and parted sidewardly before marking full impact upon the bow. The water will flow past the bow with less turbulence than would otherwise occur in the case of a submerged bulb, whereby the amount of hull resistance is greatly reduced.
When the vessel is moving through the water a damming occurs in front of the vessel, which locally raises the water level just at the bulb, so its upper periphery, in reality, will be more or less submerged, which reduces its efficiency for above stated purpose. The damming, i.e. the raising of the water, will to some extent depend upon the occasional speed of the vessel.